Add Authentik SSO via OAuth2/OIDC

Implements Single Sign-On with Authentik alongside the existing
password login. Admins configure Client ID, Client Secret and Base URL
directly in the app; the SSO button on the login page only appears
when the configuration is complete and enabled.

- SsoSetting model + migration (one-row config table)
- sso_provider_id / is_sso_user fields on users (migration)
- SsoController: redirect to Authentik + callback (token exchange,
  userinfo fetch, auto-create unknown users)
- Admin\SsoSettingController + admin/sso/show view with setup guide
  and one-click Redirect URI copy
- Admin dropdown: SSO-Konfiguration entry
- Login page: Authentik button rendered conditionally

No additional Composer packages required.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/Models/User.php

@@ -22,6 +22,8 @@ class User extends Authenticatable
         'email',
         'password',
         'is_admin',
+        'sso_provider_id',
+        'is_sso_user',
     ];
 
     /**
@@ -43,7 +45,8 @@ class User extends Authenticatable
     {
         return [
             'email_verified_at' => 'datetime',
-            'password' => 'hashed',
+            'password'          => 'hashed',
+            'is_sso_user'       => 'boolean',
         ];
     }